Little progress has been made recently in our understanding of the epidemiology of breast cancer. While results from epidemiologic studies regarding reproductive factors remain fairly reproducible from one study to another, other associations such as that between breast cancer risk and dietary fat intake, although biologically plausible, are not consistently found in direct study of humans, while yet other associations, which appear less plausible biologically, become stronger (such as the increased risk associated with modest levels of alcohol consumption). In this paper we attempt to review the epidemiology and biology of breast cancer jointly and describe possible mechanisms of breast cancer induction, the cellular composition of the breast, the epidemiology of breast cancer, and salient biological features, and attempt to reconcile the biology and epidemiology. It becomes obvious that future progress depends on better biological thinking by epidemiologists, and vice-versa. Areas of further research are suggested and discussed, concluding that the ability to measure diet with greater precision could have an important role to play in clarifying our understanding of breast cancer.